Field of the Invention
The present invention relates to a semiconductor optical modulator and an optical module, such as a semiconductor Mach-Zehnder modulator frequently used in, for example, trunk-line long distance optical communication.
Description of the Background Art
Optical modulators used in optical communication systems are roughly divided into electro-absorption (EA) modulators for modulating the intensity of light and Mach-Zehnder (MZ) modulators for modulating the phase of light. Since the MZ modulators can reduce the wavelength chirps (wavelength variations) more than by the EA modulators, the MZ modulators can be particularly used for long-distance transmission.
The MZ modulators are divided into MZ modulators (LN-MZ modulators) containing LiNbO3 (LN) using the refractive index change induced by the Pockels effect, and MZ modulators (semiconductor MZ modulators) having a semiconductor multiple quantum well structure in which the refractive index change increases by the quantum-confined Stark effect, among semiconductor materials using the refractive index change caused by the optical absorption change.
The LN-MZ modulators have less modulation loss because the optical absorption when the modulators are driven is negligible. However, they have relatively larger 2 Vπ (Vπ represents a half-wave voltage), that is, approximately 6 V at a maximum. In contrast, the semiconductor MZ modulators have relatively smaller 2 Vπ, that is, approximately 3 V at a maximum, but have larger modulation loss in transmission than the LN-MZ modulators because the optical absorption occurs when the modulators are operated. The smaller the modulation loss of the optical modulators is, the more advantageous the transmission is. Furthermore, the smaller the half-wave voltage Vπ is, the more the power consumption can be reduced. Thus, the optical modulators require smaller modulation loss and a smaller half-wave voltage Vπ. The semiconductor multiple quantum well structure is disclosed by, for example, C. Rolland et al., “10 Gbit/s, 1.56 μm MULTI QUANTUM WELL InP/InGaAsP MACH-ZEHNDER OPTICAL MODULATOR”, ELECTRONICS LETTERS, March 1993, vol. 29, no. 5, pp. 471-472 (FIG. 1) (hereinafter referred to as Non Patent Document 1).
Among the MZ modulators that can reduce the wavelength chirps (wavelength variations) more than by the EA modulators, the LN-MZ modulators and the semiconductor MZ modulators have different dependence of the wavelength chirps (wavelength variations) on voltage.
Since the refractive index change is induced by the Pockels effect in the LN-MZ modulators, generally, the refractive index linearly changes with respect to the reverse bias voltage. In contrast, since the refractive index change is induced by the quantum-confined Stark effect in the semiconductor MZ modulators, the refractive index changes not linearly but non-linearly with respect to the reverse bias voltage.
In the semiconductor MZ modulators, an α parameter that is an indicator of the chirp (wavelength variation) characteristics is not zero because of the non-linearity of the refractive index with respect to the reverse bias voltage. Thus, there is a problem in that the chirp characteristics of the semiconductor MZ modulators are more disadvantageous than those of the LN-MZ modulators.